Electrical appliances are usually connected to a power grid to be fed with the electrical power to operate them. Often, such appliances include a power supply unit to convert the electrical power delivered by the grid into a specific form of electrical power that may be used by the appliance itself or that may be further provided to another device connected to the appliance.
In order to limit undesired feedback into the grid, such appliances usually have to fulfil some requirements regarding the current drawn from the grid. These requirements for example include limitations of the harmonic content of the current drawn by an appliance such as for example defined in the standard IEC 61000-3-2. The current drawn by power supplies such as for example switch-mode power supplies usually includes a series of harmonic components where the magnitudes of these harmonic components depend on the internal design of the power supply, in particular on the design of the power circuit. Said standard for example defines maximum values for the magnitude of each harmonic component such that these magnitudes become smaller, the higher the number of the harmonic become.
Usually a (switch mode) power supply includes a front end and a back end power stage, in particular power supplies with an input power of more than 50 W (Watt). The front end is usually a PFC (power factor correction) stage which minimizes the harmonic content of the input current of the power supply and which generates a more or less constant output voltage and the back end is usually a DC/DC convertor operating from a generally constant input voltage. The PFC stage comprises both power train and control logic. The power train for example is a boost convertor located directly after a bridge rectifier of the PFC stage. The control logic adapts the input current in such a way that it immediately follows the input voltage. This means that the input impedance of the PFC essentially is a resistor. Its resistance thereby is a function of the grid voltage and the output power of the power supply.
The above topology works perfectly for a one-phase input, where the single phase is provided between a line conductor and the neutral conductor. But for a three-phase input, for example a grid having three lines but no neutral conductor it may create too high harmonics at higher power levels. Depending on the particular application and configuration, for example depending on the capacitance downstream of the rectifier, this topology may work for example to power levels up to 500 W or even up to 1500 W because of the reduced current conduction angle. To overcome this issue, a so-called passive PFC choke could be provided downstream of the three-phase bridge rectifier or an active PFC circuit can be provided which comprises one PFC choke per phase line.
The passive PFC choke solution has however limitations regarding power loss and size whereas the other solution is suitable for higher power levels only, i. e. for power levels up to approximately 2 kW.
Another drawback of the prior art solutions is that they do not actively damp the resonance circuits that are formed by the grid impedances in conjunction with the EMI capacitors that are often provided between the phase lines and the neutral conductor of the power supplies input filter. These EMI capacitors are also designated as X-capacitors. Load variations at the output of the power supply lead to input current variations which excite such resonance circuits which may lead to undesired high input voltages that may even lead to a malfunction or destruction of the power supply.